Blank Subtraction USP

[cody84]

Is subtracting the blank from mobile phase acceptable for USP?

How about using and injection of matrix and subtracting from the sample?

I mean using the software (ie. Empower)

Thanks in advance!

[krickos]

Hi

I think there is a few long threads on this topic in the data/software section.

My very breif comments on this:
-not aware of USP allowing it nor FDA or any other similar source in general

-from the ICH Q2 validation guideline:"In some cases, to obtain linearity between assays and sample concentrations, the test data may need to be subjected to a mathematical transformation prior to the regression analysis. Data from the regression line itself may be helpful to provide mathematical estimates of the degree of linearity."
My take on that is: if not tranformation of data is not included in validation it is a NO-NO as it may add to variation for example.

-Any transformation of data needs to be traceble to the orginal data (do not have reference at hand currently), so unless you use like Electronic Records or save orginal data you have another issue

Technically speaking sure detectors as they are today with our software sure do some "signal handling" as it is but wont go into that.

So all in all I would be careful to use such software options unless it has been clearly included/addressed in a validation (which you typically do not have acess to for pharmacopiea methods).

[adam]

In my opinion I would consider it an option of last resort.

If you have tried several columns, mobile phases, etc and cannot get your peak separated from a matrix or background peak, then your only remaining option is to subtract it out. In this case I would want to have two things:

- Validation to show that this gives reliable results.

- The ability to show due diligence. In other words, if you were ever questioned you would like to be able to show that you did in fact try different columns, mobile phases, etc.

[HW Mueller]

If
"Is subtracting the blank from mobile phase acceptable for USP?"
means whether it is ok to subract a peak or peaks in a blank injection from the baseline due to mobile phase, the two questions are apparently the same. So irrespective of USP or any other regulations, if you can demonstrate that what you are subtracting is a constant it should be ok, scientifically.
For instance, this sort of thing is done with a UV detector in that one zeros it. One knows that if the ratio of, as an example, MeOH to H2O and also the temp. stays the same that the baseline doesn´t produce any spurious peaks. We even know that if parameters change slowly we will get a smooth rise or fall of the baseline, that is why we use the baseline in front and after the peak as basis for the integration, and not the baseline long before the drift occurred.
It appears that if something is scientifically sound it is surpassing any regulations by far.

[krickos]

I agree that alot hinges on the documentation you make to use the approach.

As HW stated if you scientiflly can prove and document that you get consistent/reproducible results and spent reasoble effort trying to eliminate the blank peak issue (like different column brand, quality of mobile phase components/sample diluent etcetera) you are in a much better position the use the approach and defend it if questioned.